CONSTANT VOLTAGE TRANSFORMER BASICS AND TUTORIALS

CONSTANT VOLTAGE TRANSFORMER BASIC INFORMATION

What Is a Constant-Voltage Transformer?

A well-known solution for electrical “noise” in industrial plants has been the constant-voltage transformer, or CVT.

The typical components of a CVT are shown in Figure 2.8.2. The magnetic shunt on the central core has the following effects on the core’s reluctance. It reduces the reluctance of the core.

 This can be thought of as introducing more resistance in parallel to an existing resistance. The magnetic shunt in the CVT design allows the portion of the core below the magnetic shunt to become saturated while the upper portion of the core remains unsaturated.

This condition occurs because of the presence of the air-gap between the magnetic shunt and the core limbs. Air has a much higher reluctance than the iron core.

Therefore, most of the flux passes through the lower portion of the core, as shown by the thick lines in Figure 2.8.2.

In terms of an electrical analogy, this configuration can be thought of as two resistances of unequal values in parallel.

The smaller resistance carries the larger current, and the larger resistance carries the smaller current.

The CVT is designed such that:

• The lower portion of the central limb is saturated under normal operating conditions, and the secondary and the resonating windings operate in the nonlinear portion of the flux-current curve.

• Because of saturation in the central limb, the voltage in the secondary winding is not linearly related to the voltage in the primary winding.

There is consonance between the resonating winding on the saturated core and the capacitor. This arrangement acts as a tank circuit, drawing power from the primary. This results in sustained, regulated

oscillations at the secondary with the applied line frequency.

POLARITY TEST OF SINGLE PHASE TRANSFORMER BASIC AND TUTORIALS

SINGLE PHASE TRANSFORMER POLARITY TESTS BASIC INFORMATION

What Are The Polarity Tests Of Single Phase Transformers?

Polarity tests on single-phase transformers shall be made in accordance with one of the following methods:

a) Inductive kick

b) Alternating voltage

c) Comparison

d) Ratio bridge

Polarity by inductive kick

The polarity of transformers with leads arranged as shown in may be determined when making resistance measurements as follows:

a) With direct current passing through the high-voltage winding, connect a high-voltage direct-current voltmeter across the high-voltage winding terminals to obtain a small deflection of the pointer.

b) Transfer the two voltmeter leads directly across the transformer to the adjacent low-voltage leads, respectively.

NOTE—For example, in Figure 5, the voltmeter lead connected to H1 will be transferred to X2 as the adjacent lead,and that connected to H2to X1.

c) Break direct-current excitation, thereby inducing a voltage in the low-voltage winding (inductive kick), which will cause deflection in the voltmeter. The deflection is interpreted in d) and e) below.

d) When the pointer swings in the opposite direction (negative), the polarity is subtractive.

e) When the pointer swings in the same direction as before (positive), the polarity is additive.

Polarity by alternating-voltage test

For transformers having a ratio of transformation of 30 to 1 or less, the H1 lead shall be connected to the adjacent low-voltage lead (X1 in Figure 6).

Any convenient value of alternating voltage shall be applied to the full high-voltage winding and readings taken of the applied voltage and the voltage between the right-hand adjacent high-voltage and low-voltage leads.

When the latter reading is greater than the former, the polarity is additive. When the latter voltage reading is less than the former (indicating the approximate difference in voltage between the high-voltage and low-voltage windings), the polarity is subtractive.

Polarity by comparison

When a transformer of known polarity and of the same ratio as the unit under test is available, the polarity can be checked by comparison, as follows, similar to the comparison method used for the ratio test.

a) Connect the high-voltage windings of both transformers in parallel by connecting similarly marked

leads together.

b) Connect the low-voltage leads, X2, of both transformers together, leaving the X1 leads free.

c) With these connections, apply a reduced value of voltage to the high-voltage windings and measure the voltage between the two free leads.

A zero or negligible reading of the voltmeter will indicate that the relative polarities of both transformers are identical.

An alternative method of checking the polarity is to substitute a low-rated fuse or suitable lamps for the voltmeter. This procedure is recommended as a precautionary measure before connecting the voltmeter.

Polarity by ratio bridge

The ratio bridge can also be used to test polarity. A bridge using the basic circuit below may be used to measure ratio.